Electricity transmission networks are based on designs dating back to the early 20th century. In recent times, as the demand for electricity has increased and more power is generated from dispersed, renewable energy sources, high voltage transmission networks are approaching full capacity with increasing risks of overloading certain routes. The IEC standards define high voltage (HV) as greater than 1 kV. Typically, high voltage transmission lines operate at voltages in excess of 100 kV, perhaps several hundred kV. Typical voltages used in power networks are 275 kV and 400 kV but voltages up to 1 MV or more may be used.
Although it is possible to build new networks, this is costly, time consuming and can meet opposition from local residents. An alternative to installing a new power system would be to upgrade power carried by the existing networks by either increasing the voltage or the current for the existing transmission lines. Increasing the current on an existing circuit can lead to an increased amount of conductor sag through conductor heating and this can cause infringement of ground clearance regulations. Increasing the voltage on an existing circuit will cause a greater risk of flashover (short circuit to earth or between phases) and may result in the system being in breach of statutory regulations. Either solution will also lead to increased electromagnetic fields at ground level. The requirements for minimum ground clearances, a reliable system (i.e., one that does not flashover) and limits on the electromagnetic field strength at ground level mean that the possibility of increasing voltages or currents for existing tower designs is limited. These issues also limit the ability to make existing tower designs more compact. In common usage, support towers for high voltage power systems are also referred to as pylons.
A conventional tower has a body made of steel, with steel cross-arms, usually fabricated from L-section high tensile steel members. The conductors are held suspended from the ends of cross-arms by insulators. For a 275 kV system, the insulators would typically be about 2.5 m in length. The insulators hold the suspended conductors under tension and keep them spaced from the tower body and from the cross-arm itself. The height of the lowest cross-arm above ground level must take into account sag of the transmission lines due to the combined effects of ambient temperature, temperature rise from solar radiation and temperature rise from resistive heating. The electromagnetic field levels at ground level generated from the transmission conductors must also be considered. It is also important that the conductors are held high enough above ground that the minimum statutory clearance is not breached. The length of the cross-arm must be sufficient to ensure that conductor swing (through wind-driven oscillations of the insulator) does not lead to the conductors coming into close proximity to the tower body, ensuring a low risk of flashover to the pylon body. In addition to being capable of bearing the weight of the transmission conductors, the cross-arm must also be strong enough to bear the additional loads that may arise, for instance, as a result of conductor icing, wind loads, conductor breakage (which could lead to high lateral forces on a cross-arm) or a combination thereof.
Insulators used for suspending conductors from pylon cross-arms are typically provided with sheds spaced along their length in order to increase the creepage path (i.e., the shortest distance between the ends of the insulator measured over the surface). In order to inhibit current leakage by surface conduction, a ratio of creepage distance to insulator length of at least 2 is desirable.
It is desirable to be able to increase the voltage rating or current rating for power transmission networks without the need to replace existing tower bodies and without increasing the risk of flashover. It is also desirable to reduce electromagnetic field strength at ground level near towers. It is also desirable to be able to design tower bodies that are more compact than existing designs.
British Patent GB 1,034,224 discloses insulators used as structural members of cross-arms or frames for supporting overhead line conductors from poles or towers. The insulator disclosed comprises two or more separate rods of resin-bonded fiber spaced apart along their length by mutually spaced insulating cross-members jointed to the rods at intervals in order to restrain the rods from buckling under compressive loads. The rods are disclosed as being of square or circular cross section.
Japanese patent publication JP06-335144 discloses the use of cross-arms as insulators for transmission lines, with a number of arm members combined into a truss arrangement. The cross-arms disclosed are circular or hollow in cross-section.